1.2 What is a Kerberos Principal?

A Kerberos principal is a unique identity to which Kerberos can assign tickets. Principals can have an arbitrary number of components. Each component is separated by a component separator, generally `/'. The last component is the realm, separated from the rest of the principal by the realm separator, generally `@'. If there is no realm component in the principal, then it will be assumed that the principal is in the default realm for the context in which it is being used.

Traditionally, a principal is divided into three parts: the primary, the instance, and the realm. The format of a typical Kerberos V5 principal is primary/instance@REALM.

• The primary is the first part of the principal. In the case of a user, it's the same as your username. For a host, the primary is the word host.
• The instance is an optional string that qualifies the primary. The instance is separated from the primary by a slash (/). In the case of a user, the instance is usually null, but a user might also have an additional principal, with an instance called admin, which he/she uses to administrate a database. The principal jennifer@ATHENA.MIT.EDU is completely separate from the principal jennifer/admin@ATHENA.MIT.EDU, with a separate password, and separate permissions. In the case of a host, the instance is the fully qualified hostname, e.g., daffodil.mit.edu.
• The realm is your Kerberos realm. In most cases, your Kerberos realm is your domain name, in upper-case letters. For example, the machine daffodil.example.com would be in the realm EXAMPLE.COM.